The invention relates to an automatic speed ratio control system which controls the speed ratio of an stepless transmission of an automotive vehicle so that a given engine condition is achieved.
The output torque T.sub.E of an engine is related to the torque which is dissipated by the running of the vehicle as the following equation, assuming no torque loss: EQU T.sub.E =(J.multidot.n.sub.2 +A.multidot.n.sub.2.sup.2 +B).times.e (1)
where "e" represents a speed ratio, "n.sub.2 " represents the rotational speed of an output shaft, n.sub.2 represents dn.sub.2 /dt, "J" represents the output inertia, "A" represents windage loss and "B" represents grade resistance. Since the speed ratio "e" presents the ratio of the rotational speed "n.sub.2 " of the output shaft to that "n.sub.1 " of the input shaft, we have EQU n.sub.2 =e.multidot.n.sub.1 ( 2)
Substituting the equation (2) into the equation (1) and solving for the rate of change "e" of speed ratio, we have ##EQU1## Ideally an automatic speed ratio control system should provide the rate "e" represented by the equation (3) as an output for controlling the speed ratio. While this may appear to be practicable, it is impossible in practice to achieve such an arrangement.
A conventional arrangement has been an automatic speed ratio control system of an integrating servo type which comprises a desired value signal generator which produces a signal indicative of a desired value of the rotational speed of an engine which corresponds to the magnitude of an engine throttle opening or an engine output torque or which produces a signal indicative of a desired value of the engine output torque which corresponds to the rotational speed of the engine, a detector providing a signal indicative of either the rotational speed or the output torque of the engine, a control signal generator for producing a control signal in accordance with a deviation of the detector signal from the desired value signal, and a controller responsive to the control signal to change the speed ratio of a stepless transmission.
In such an arrangement, the rate "e" is represented by either equation (4) or equation (5). EQU e=K.sub.1 (n.sub.o -n.sub.E) (4)
where "K.sub.1 " represents a constant, "n.sub.o " a desired value of the rotational speed of the engine and "n.sub.E " the rotational speed of the engine. EQU e=K.sub.2 (T.sub.o -T.sub.E) (5)
where "K.sub.2 " represents a constant, "T.sub.o " a desired value for the output torque of the engine and "T.sub.E " the output torque of the engine.
When an arrangement is made producing the rate "e" as represented by the equation (4) and in which the value of the constant "K.sub.1 " is chosen to satisfy a normal running it is found that an abnormal rise in the rotational speed of the engine occurs during a rapid starting. A study of such situation revealed that this is due to the fact that the rate "e" is not in substantial inverse proportion to the speed ratio "e" when the equation (3) dictates that it should be under a condition where the magnitude of the speed ratio "e" is small, and also due to an increase in the rotational speed of the engine which occurs prior to an increase in the output torque of the engine during the starting phase and to a lag involved with the controller. These factors are also present when the rate "e" is represented by the equation (5).
Therefore, it will be seen that the elimination or alleviation of the described difficulty requires that an increase in the speed ratio occurs in immediate response to an increase in the throttle opening and that the rate "e" be substantially inversely proportional to the speed ratio "e".